(a) Field of the Invention
The present invention relates to a liquid crystal display, and in particular, to a gamma voltage generator of a liquid crystal display (LCD) that is capable of removing a residual image by compensating a gamma voltage.
(b) Description of the Related Art
Typically, a liquid crystal display uses a thin film transistor as a switching element for applying an analog gray voltage to a pixel so as to display an image. The number of the gray voltages is limited to 64 or 256 according to the types of digital analog converter (DAC) provided in a source driver. The DAC produces 64 or 256 gray voltages by selectively switching 6-bit or 8-bit red (R), green (G), and blue (B) digital data from an external source, and supplies the gray voltages to the pixels via data lines in an LCD panel assembly.
FIG. 1 is an equivalent circuit diagram of a typical pixel; and FIG. 2 is a graph showing typical waveforms of a gate voltage, a data voltage, and a pixel voltage.
A gray voltage generated by a DAC for supply to a data line is expressed as a data voltage Vdata in FIG. 1 and FIG. 2. The data voltage Vdata becomes a pixel voltage Vp after passing through a TFT which is turned on by a high state VgH of a gate voltage Vg. The voltage difference between the pixel voltage Vp and a common voltage Vcom applied to a liquid crystal capacitor Clc determines the transmittance of light. Since the common voltage Vcom has a fixed value or swings between two fixed values, the pixel voltage Vp substantially determines the light transmittance.
Under the high value VgH of the gate voltage Vg of the TFT, the pixel voltage Vp reaches the data voltage Vdata. The pixel voltage Vp drops by as much as a kickback voltage Vk due to parasitic capacitors (Cg, Cgd) after the gate voltage Vg becomes low VgL.
The kickback voltage Vk is determined by the following equation:
      Vk    =                                        (                          Vcom              -              Vp                        )                    ⁢                      (                          Clcon              -              Clcoff                        )                          +                  Δ          ⁢                                          ⁢          VgCgd                +                              (                          VgH              -              Vp                        )                    ⁢                      Cg            /            2                                      Cgd        +        Clcoff        +        Cst              ,where Clcon is the capacitance of a charged liquid crystal capacitor when the pixel is charged, Clcoff is the capacitance of a completely discharged liquid crystal capacitor, Cg is a parasitic capacitance between a channel and a gate of the TFT, and Cgd is a parasitic capacitance between the gate and a drain of the TFT.
As shown by the equation, the kickback voltage Vk varies significantly depending on the voltage difference between the pixel voltage Vp and the common voltage Vcom, as shown in FIG. 4, as well as depending on the pixel voltage Vp itself. It is because the capacitance of the liquid crystal capacitor Clc depends on the voltage across the liquid crystal capacitor Clc due to the dielectric anisotropy of liquid crystal. FIG. 3 shows the dielectric constant which increase as the magnitude of the bias voltage across the liquid crystal capacitor Clc. Therefore, it is hard to compensate the kickback voltage Vk using the gray voltages.
To prevent the typical distortion of the pixel voltage Vp due to the kickback voltage Vk, it is suggested that the intermediate grays where the pixel voltages Vp are about 1.8V are compensated by adjusting the common voltage Vcom, although the white and the black grays are not completely compensated. However, when an image including black and white grays is displayed for a long time, and thus a DC bias voltage having a value as much as the difference between the kickback voltage Vk and the intermediate gray voltage is applied for a long time, this causes a defect in the LCD panel assembly referred to as image sticking.